Sizing elbows when transitioning between different pipe fittings

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Dimensioning elbows when transitioning between different pipe diameters – a complete technical guide

Anyone involved in the installation of heating systems will sooner or later encounter a situation where it is necessary to connect two pipes of different diameters while also changing the direction of the pipe. At this very moment, most plumbers reach for either a combination of a standard elbow and a reducer or consider the use of a specialized reduced elbow. This seemingly simple choice, however, has a significant impact on pressure losses, the hydraulic performance of the entire system, installation time, and the overall reliability of the installation.

This article focuses on the issue of dimensioning elbows when transitioning between different diameters from the perspective of hydraulics, material, spatial constraints, and economic aspects. We draw from everyday practice – from dozens of projects where the wrong choice of fitting caused unnecessary problems, as well as those where proper dimensioning saved the customer time, money, and nerves.

Why does a change in pipe diameter occur in heating distribution systems

Before we dive into the actual dimensioning, it is important to understand why a change in the cross-section of the pipe occurs in heating systems. It is not just a matter of installation compromises – a change in diameter is intentional and hydraulically justified in a well-designed system.

The main reasons for changing the diameter in practice are as follows:

Reducing the diameter as you move away from the heat source. The main distribution loop coming out of the boiler or manifold has a larger diameter because the maximum flow passes through it. At the branches to individual consumers (radiators, floor heating loops, storage tanks), the flow gradually decreases, and the pipe diameter must correspond to this.
Connecting devices with a fixed connection size. Circulation pumps, valves, manifolds, or boilers have a defined connection diameter. A distribution from a larger pipe must be reduced to match this size.
Space constraints and aesthetics. In parts of the installation where the piping is visible or runs, for example, in a raceway, it may be intentional to switch to a smaller diameter for reasons of aesthetics or space availability.
Combining systems of different generations or materials. During renovations, we often encounter situations where existing copper or steel "22" must be connected to a plastic distribution "15" for a new radiator.

Changing the diameter is therefore not a mistake in the design – it is a common part of every real installation. The problem arises only when this change is not done correctly, or when it is combined with a change in the direction of the pipe using an unsuitable fitting.

Hydraulic basics – what happens with the flow when the cross-section changes

Proper dimensioning of an elbow when transitioning between different diameters requires a basic understanding of hydraulics. It is not complicated physics – knowing a few practical principles is enough, which are repeatedly confirmed in practice.

Flow velocity and cross-section

The basic law of mass conservation (continuity) states that with a constant flow rate, the flow velocity is inversely proportional to the cross-sectional area of the pipe. If we move from a DN22 pipe (internal diameter approx. 20 mm, area approx. 314 mm²) to a DN15 pipe (internal diameter approx. 13 mm, area approx. 133 mm²), the flow velocity increases by approximately 2.4 times. In practice, this means significantly higher noise and increased pressure losses in the narrower section.

Recommended flow velocities in heating distribution systems are usually in the range of 0.3–0.7 m/s for main lines and 0.2–0.5 m/s for branches to consumers. Exceeding these values leads to noise (characteristic noise in the pipes), increased wear and lower hydraulic efficiency. That is why the rule of thumb is: the smaller the diameter, the smaller the flow, and the entire network design must be adapted accordingly.

Pressure losses in an elbow with a reducer

Each fitting introduces a local pressure loss into the network. An elbow itself has a higher resistance than a straight section of pipe – it depends on the angle (45° or 90°) and the diameter. When we add a change in diameter to the elbow, the pressure losses add up, but not simply – it depends on the direction of the transition (narrowing vs. widening) and the internal geometry of the fitting.

In practice, for standard residential installations (flow up to 0.5 m/s, pressure losses up to 100 Pa/m), the difference between a quality reduced elbow and a combination of an elbow and a reducer is usually in the range of 10–30 Pa per connection. This may seem negligible, but with ten such connections in the system, it can add up to 100–300 Pa – which is significant when selecting a pump.

HEPWORTH system sizing advice – what is available and what you really need

The HEPWORTH system works with standard pipes compatible with European copper installation standards. The most common diameters in apartment and residential installations are 15 mm, 22 mm, and 28 mm. These three sizes cover the vast majority of common cases:

DN15 (15 mm) – connection to radiators, end sections of branches, connection of thermostatic valves
DN22 (22 mm) – main branches within an apartment or floor, connection to manifold in smaller systems
DN28 (28 mm) – main risers, primary loop in larger houses, connection to boiler

When designing a layout, it is key to know where transitions between these diameters occur and whether this transition is associated with a change in direction. Precisely the combination of both requirements – change of diameter and change of direction – is a situation for which reduced elbows are designed.

Typical diameter combinations in practice

From practice, the most common transitions are:

22 → 15 mm: By far the most common case. The main branch in the apartment (22 mm) is extended to a specific radiator with a 15 mm connection. The transition is almost always associated with a change in direction – the pipe usually runs horizontally and must go vertically up or down to the radiator.
28 → 22 mm: A riser or main loop is reduced to a distribution branch. Less common in solutions with soldering, more common in pressed systems.
22 → 15 mm with threaded end: Direct connection to a radiator thermostatic valve, which usually has a 1/2" thread. This is a case for Soldered elbow 1/2" x 15, which in one piece solves the diameter change, direction change and valve thread.

When to use a reduced elbow and when to prefer separate fittings

This question divides plumbers into two camps. Some swear by reduced elbows always, others use them only exceptionally. The truth, as usual, lies somewhere in the middle – it depends on the specific case.

Reduced elbow is suitable when:

Change of diameter and change of direction occur at the same place. If you need to go from DN22 to DN15 and at the same time turn 90°, a reduced elbow is simply the most elegant solution with fewer joints and lower pressure losses.
Space is limited. A combination of elbow and reducer takes up more physical space. In a pipe chase, floor construction or behind a radiator, every centimeter may be important.
You want to minimize the number of joints. Every joint is a potential leak point. Fewer joints = lower risk of failure.
You are connecting directly to a threaded fitting. Soldering integrated in the elbow (e.g. 1/2" x 15 mm) eliminates the need for an additional adapter and at the same time maintains the compactness of the whole unit.

Separate fittings (elbow + reducer) are more suitable when:

Change of diameter and change of direction occur at different places. If the DN22 pipe runs straight for 30 cm and only then turns to DN15, it makes sense to use a reducer where the transition is and an elbow where the direction changes.
You are working with average combinations for which there are no available reduced elbows. For example, the transition 28 → 15 is not a standard product – there is no other way than reducer 28/22 + reducer 22/15 or a combination with an elbow.
You are repairing an existing installation in parts. If the pipe route is already given and you are only changing a specific section, the flexibility of separate fittings can be an advantage.

More about this decision can be read in the article Reduced elbow or reducer separately – what is more cost-effective, where we compare both approaches also from an economic point of view.

Practical procedure for elbow sizing when changing diameters

Sizing does not have to be complicated. The following practical procedure, verified by experience, will help even a less experienced plumber make the right decision:

Step 1 – Determine the flow in the given section

Before selecting a diameter, you must know what flow will go through the branch. In a standard residential installation, we start from the thermal output of the appliance. Roughly: with a temperature difference of 20 K (e.g. 70/50 °C) and a flow of 1 l/min, you transfer about 1.4 kW. For a radiator with a power of 1.4 kW, you therefore need a flow of about 1 l/min. For a power of 2.8 kW it will be 2 l/min, etc.

Step 2 – Choose a diameter for the desired speed

Flow speed is a key parameter. For orientation, we list the maximum recommended flows at a speed of 0.5 m/s:

DN15 (internal ⌀ ~13 mm): max. approx. 40 l/h → suitable for outputs up to ~1.5 kW at ΔT 20 K
DN22 (internal ⌀ ~20 mm): max. approx. 95 l/h → suitable for outputs up to ~3.7 kW at ΔT 20 K
DN28 (internal ⌀ ~26 mm): max. approx. 160 l/h → suitable for outputs up to ~6.2 kW at ΔT 20 K

If the flow in the branch corresponds to DN15, but you are coming from DN22 (main branch), this is precisely where you need a reducer – and if a change of direction is also needed, reach for a reduced elbow.

Step 3 – Assess the route geometry

Find out whether the change of diameter and the change of direction occur at the same place or at different places. If at the same place – reduced elbow. If at different places – separate fittings. This rule applies in 90 % of cases, the rest you solve according to product availability and space conditions.

Step 4 – Check the direction of the reduction

An important detail that is often overlooked in practice: a reduced elbow always has a larger end on one side and a smaller end on the other. During installation, the larger end must point toward the larger pipe and the smaller end toward the smaller one. This may sound obvious, but in tight spaces or when working in a rush, this basic detail can be messed up, which may require disassembling the entire assembly.

Step 5 – Check the availability of the product in the correct angle

Reduced elbows are produced in the standard angle of 90°. If you need a 45° angle, the solution is usually a 45° elbow with an additional reduction on one side (such products are less common than their reduced counterparts). For example, 45° Elbow – 15 x 15 or 45° Elbow – 22 x 22 are non-reduced variants for the same diameters – if you need a 45° angle with a diameter transition, you can combine them with a reduction on the inlet side. Learn more about choosing the correct angle in the article 45° Elbow or 90° Elbow – When to Use Which Type.

Special case: elbow with threaded connection during diameter transition

A special category includes elbows where one end has a crimped sleeve for a plastic pipe and the other end is threaded (external or internal). This combination is very practical when connecting to fittings – thermostatic valves, ball valves, flow regulators, and similar components.

A typical example from practice is connecting a radiator: the supply line is in DN15, and the thermostatic valve has a G1/2" (1/2" external thread) connection. Instead of combining a DN15×15 elbow with a 15 mm to 1/2" internal thread adapter, a much cleaner solution is Elbow with a 1/2" x 15 Thread. This single component replaces two fittings and one additional connection, resulting in a more compact and reliable solution.

When selecting an elbow with a thread, make sure to check:

Thread size: Most commonly 1/2" (G1/2), less frequently 3/4" (G3/4). It depends on the fitting you are connecting to.
Type of thread – external vs. internal: Radiator valves typically have an external thread (BSP/G), so the elbow must have an internal thread to screw onto it. This is a subtle detail where even experienced plumbers can make mistakes – if in doubt, read the article External vs. Internal Thread on an Elbow – What Difference Does It Make?.
Pipe side diameter: It must match the diameter of the supply pipe (15 mm, 22 mm, etc.).

Common mistakes in dimensioning and installation of reduced elbows

Experience with dozens of customer projects shows the same mistakes being repeated. Knowing them can save you time and money:

Mistake 1 – Overdimensioning the diameter at low flow rates

The customer (or an inexperienced plumber) does not want to risk using a "small pipe" and keeps DN22 all the way to the radiator, where DN15 would suffice. The result is low flow velocity, air tends to settle in the pipe more easily, and problems with air venting arise. Additionally, there is unnecessary material consumption and higher heat loss through the pipe.

Mistake 2 – Underdimensioning in a branch with multiple consumers

The opposite case: the plumber sees that DN15 is sufficient for one radiator and installs DN15 on the entire branch, which connects five radiators. At 5 × 1.5 kW = 7.5 kW, the flow through this branch is almost 5 l/min, which in DN15 means a velocity of about 2.5 m/s – significantly above the recommended limit. The result is noise, vibrations, and rapid wear of seals.

Mistake 3 – Intentional creation of "over-pumping" through a reduction

Some people try to save pressure difference by inserting a reduction to a smaller diameter in a branch to increase resistance in that branch and hydraulically balance the system. This is an inappropriate way of hydraulic balancing – for this purpose, there are regulating fittings (regulating valves, balancing valves). Using a reduction as a hydraulic resistance is imprecise, uncontrollable, and will stop working when system parameters change (boiler replacement, adding a new circuit).

Mistake 4 – Installation without checking the flow direction

When reducing (larger → smaller diameter), the pressure loss is relatively small. When expanding (smaller → larger diameter), the pressure loss is usually lower, but there is a risk of flow separation and turbulence if the internal geometry of the fitting is not suitable. Reduced elbows are mostly designed for one preferred flow direction – check the product's technical sheet if you have any doubts.

Mistake 5 – Confusing crimping diameters when combining systems

The HEPWORTH system (plastic piping) and international measuring systems for copper work with nominal diameters, where the pipe's outer diameter corresponds to the nominal size (15 mm outer ⌀ for copper = 15 mm crimp sleeve for HEPWORTH). Problems arise with other plastic systems, where for example, a 16 mm PEX pipe has a 16 mm outer ⌀, but the crimp sleeve of the system is different. Always check system compatibility before mixing different manufacturers.

Concrete examples from practice – real customer situations

Example 1: Reconstruction of apartment heating, Bratislava, 3-room apartment

The customer had an old cast iron system, which we completely replaced with a panel and a condensing boiler, along with ten radiators. The main supply line was run in DN22 from a manifold located in the entrance hall. Each radiator was connected via a DN15 branch. At each branch-off point, we needed to solve the transition from DN22 → DN15 combined with a change in direction.

In the kitchen and living room, the conditions were simple – the pipe ran horizontally along the baseboard and had to go up to the radiator. We used a DN22 T-piece on the main branch and a reduced 22×15 elbow on the vertical outlet. In the bathroom, the space was much tighter, and we also had to transition directly from the riser to the radiator's thermostatic valve with a 1/2" connection. Here, we chose Elbow with a 1/2" x 15 Thread, which connected the end of the DN15 pipe directly to the valve in one piece, without an additional adapter.

Example 2: New Construction of a Single-Family House, Nitra, Floor Heating + Radiators

In this project, the primary circuit was dimensioned to DN28 from the boiler to the manifold. From the manifold, floor heating loops were run in PE-RT pipes of 16 mm and at the same time, bathroom radiators were connected via DN15 HEPWORTH. When connecting the radiator circuit from the manifold, we had to solve the transition from the manifold port of 3/4" to DN15. Here we used a combination of a 3/4" (internal thread) × 15 mm reducer and a 15×15 elbow. An alternative would have been an elbow with a direct 3/4"×15 reducer, but this combination was not available in the correct angle at that time, so we opted for a two-piece solution.

Example 3: Repair of a Fault, Old Panel Apartment, Košice

The customer called to say that a connection at the radiator was leaking. Upon arrival at the site, it turned out that the previous plumber had used the following "construction" at one point: an elbow DN22 + reducer 22/15 + adapter 15 mm × 1/2" (internal) + thermostatic valve. The entire assembly was 14 cm long and the sealing in the reducer adapter was crushed during installation. The result was a direct water passage behind the thermostatic valve. We replaced the entire assembly with a single elbow with a 1/2"×15 compression, thus eliminating three connections and shortening the entire assembly by 12 cm. Since then, no problems have occurred.

Choosing the Right Product – Quick Overview by Situation

For quick orientation, we present a table of the most common situations and recommended products:

Situation	Recommended Solution	Product Example
DN15 → DN15, 90° direction change	Same diameter elbow	Elbow 15 x 15
DN22 → DN22, 90° direction change	Same diameter elbow	Elbow 22 x 22
DN22 → DN15, 90° direction change, pipe on both sides	Reducing elbow 22×15, 90°	Reducing elbow from the category
DN15 → thermostatic valve 1/2", 90° direction change	Elbow with compression 1/2"×15	Elbow with compression 1/2" x 15
DN15 → DN15, 45° direction change	45° elbow, same diameter	45° elbow – 15 x 15
DN22 → DN22, 45° direction change	45° elbow, same diameter	45° elbow – 22 x 22
DN22 → DN15, 45° direction change	45° elbow + reducer, or reducer + 45° elbow	Combination of separate fittings

Material Compatibility and Long-Term Reliability

When choosing a reducing elbow, you should also consider the material of the pipes you are connecting. The HEPWORTH system is designed for plastic piping (polybutylene, PB), where crimping ensures a secure and tight connection without the need for soldering. When connecting to a copper pipe of the same diameter, compatibility is guaranteed by the same external dimensions.

If you are combining HEPWORTH with older steel piping, you must always use threaded adapters – plastic crimp rings cannot be applied to steel. In this case, an elbow with a thread on one side (e.g. 1/2"×15) is the correct choice for connecting to a threaded valve or a steel pipe with a threaded end.

The long-term reliability of the HEPWORTH system's crimped connection depends primarily on correct installation: a clean pipe cut, correct insertion depth (must be visible mark or stop), and the use of the correct crimping tools (crimping dies matching the diameter). For information on leakage problems and how to fix them without replacing the entire joint, read the article Why the elbow is leaking and how to fix it without replacing the entire joint.

Most Frequently Asked Questions (FAQ)

Can I use a reducing elbow in the opposite direction – from smaller to larger diameter?

Hydraulically, it is possible, but it is not recommended as a standard approach. Most reducing elbows have a conical internal geometry optimized for flow in the narrowing direction (larger → smaller). In the opposite direction (expansion), greater turbulence occurs in the arc where the flow changes direction. The result is higher local losses and potentially higher noise. If you need an expansion combined with a direction change, use an elbow + reducer arranged before (or after) the elbow in the correct order.

What impact does a diameter reduction have on the hydraulic balancing of the system?

Reducing the diameter increases the hydraulic resistance of the branch. This can be desirable if the branch is hydraulically underbalanced (too much flow is passing through it at the expense of other branches), but it is an imprecise and unregulated method of balancing. The correct approach is to use regulating valves (thermostatic regulating valves, balancing valves) and design the diameters according to a hydraulic calculation so that the resistances are naturally balanced. Diameter reduction should not be an intentional tool for hydraulic balancing.

How can I determine if an elbow is suitable for my HEPWORTH pipe if there is no marking?

By measuring the outer diameter of the pipe with a caliper. A HEPWORTH 15 mm pipe has an outer diameter of 15 mm, a 22 mm pipe has an outer diameter of 22 mm, etc. This is in line with the European standard for copper installation, with which HEPWORTH is compatible. If the caliper shows a different dimension (e.g. 16 mm or 20 mm), it is likely a different system (PEX, PE-RT, multilayer pipe), and it is necessary to verify the compatibility of the crimp rings. Read more in the article How to determine the correct diameter and angle of the elbow for your system.

Is the difference in pressure loss between a reducing 90° elbow and a combination of a 90° elbow + reducer actually measurable in practice?

Yes, the difference is measurable, but its practical impact depends on the number of such connections in the system and the overall hydraulic design. For a single connection, the difference is in the order of 10–30 Pa, which is negligible. For ten such points, we are talking about an additional 100–300 Pa – this is equivalent to extending the pipe run by several meters. For a properly dimensioned system with a suitable pump, this is not a problem, but in so-called "borderline" cases, where the pump is operating at the limit of its capacity, this difference can cause insufficient heat supply to distant consumers.

Can I combine plastic HEPWORTH elbows with copper pipes on both sides?

HEPWORTH elbows are primarily intended for connecting with plastic pipes (PB – polybutylene) of the HEPWORTH system. The outer diameter of the pipe corresponds to the standards for copper (15, 22, 28 mm), so physically, the fitting can be pushed onto a copper pipe as well. Some types of HEPWORTH elbows are certified for use with copper – check this for the specific product. Threaded ends (elbow with compression) are naturally compatible with any threaded counterpart, regardless of the material.

What is the recommended pressure and temperature for reducing elbows in the HEPWORTH system?

The standard parameters for the HEPWORTH system for residential heating are a maximum operating pressure of 10 bar (1.0 MPa) and a maximum medium temperature of 95 °C. For normal low-temperature heating (floor loops at 35–45 °C, radiator circuits at 55–70 °C), these parameters are well within the safety margin. Always check the technical data sheet of the specific product, as some plastic fittings may have limitations when combining high temperature and pressure.

Conclusion – dimensioning elbows when changing diameter is not a coincidence

The correct selection and dimensioning of an elbow when transitioning between different pipe diameters is more than just an installation detail. It affects the hydraulic efficiency of the entire system, the reliability of the installation, the spatial compactness of the fittings, and ultimately the customer's satisfaction with the heating system. The basic rules are simple: dimension the diameter according to the flow, reduce it where the flow decreases, and if the change in diameter coincides with a change in direction, choose a reduced elbow instead of a combination of two fittings.

The HEPWORTH system offers a sufficient range of products for standard residential installations to solve practically all standard situations. In the case of non-standard combinations of diameters or angles, a combination of separate fittings may sometimes be more advantageous – it is important that each connection is made correctly, with a clean cut and the correct insertion depth. A complete overview of available products can be found in the category of reduced elbows.

Do you have a question on this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.